A typical application for a transmission mechanism of the type to which this invention relates is in a bag making machine or a similar machine wherein a web passes through the nip of a pair of feed rollers that rotate intermittently. In the case of a bag making machine the web is usually tubular but flattened, and during each period of rotation of the feed rollers is advanced towards a station at which its opposite walls are sealed to one another across its width and at which it is cut through to form an individual bag. Sealing and severing occur during the pauses between web advances. Each advance of the web carries it through a distance equal to the length of a single bag, and bag length is thus controlled by control of feed roller rotation.
The power source for the feed rollers comprises a steadily rotating driving shaft. The feed rollers are intermittently connected with that shaft by means of a clutch-brake mechanism that is actuated in synchronism with rotation of the driving shaft in a recurrent operating cycle.
In the usual bag making machine arrangement, the driving shaft drives an input member through a crank and pitman connection whereby the input member is rotated in one direction during one half of each driving shaft revolution and is rotated in the opposite direction during the remainder of the operating cycle. The feed rollers are drivingly connected with an output member that is clutched to the input member while the input member rotates in one direction and is declutched and braked during opposite rotation of the input member. Because input member acceleration and deceleration in each direction can be plotted as a simple harmonic motion, the output member can be clutched to the input member and declutched from it at the instants when the input member goes through zero speed during reversal of its direction of rotation, and the feed rollers are accelerated and decelerated smoothly during each period of their rotation.
It will be apparent that for precisely uniform increments of web advance, a clutch-brake mechanism of the character described must have very fast and precisely timed actuation whereby the clutch elements will be engaged and disengaged during the infinitesimal time interval of zero speed of the input member. It will also be apparent that the clutch elements must be engaged under substantial axial biasing force, to enable them to transmit the high torque needed for acceleration and deceleration of the feed roller masses. To meet these requirements, clutch-brake mechanisms for bag making machines have heretofore had relatively powerful electromagnetic or pneumatic actuators for effecting the axial shifting needed for engagement and disengagement of the clutch elements and braking elements. Electromagnetic clutch brake mechanisms were commonly used. A typical mechanism of that type is dislosed in U.S. Pat. No. 2,997,889, to Schjeldahl et al.
A prior electromagnetic actuator had the known disadvantage that there was an inherent delay in its response to inputs that commanded shifting from the braking mode to the clutching mode and vice versa. In the case of shift from the braking mode to the clutching mode, the clutch elements could be engaged with reasonable promptness by overexcitation of the electromagnet that actuated them, but due to slow decay in the magnetic field of the other electromagnet, the brake tended to remain engaged as the clutch elements began to rotate. A similar torque overlap, occurring in the shift from clutching to braking mode, could cause a slight rearward rotation of the feed rollers. Such torque overlaps were inevitably accompanied by some slippage between engaged clutch elements and braking elements, all of which elements were friction plates. Adjustments could be made, to compensate for inherent response delays and for most of the slippage, but there were both short term and long term variations in slippage. The heating of the friction plates that resulted from slippage reduced their torque transmitting capability and increased their slippage. Of course any wear on the friction plates also tended to cause slippage. The end result was a need for frequent and relatively costly adjustment and service on prior bag making machines having clutch-brake mechanisms.
It is well known that there is normally no slippage in a clutch wherein each clutch element has circumferentially spaced teeth that project axially towards the other clutch element and interfit between its teeth. Such toothed clutch elements, like friction plate clutch elements, must be engaged under substantial bias in order to transmit a significant amount of torque. However, unlike friction plates, they have to be brought together rather gently because if they meet under high impacting force when their teeth are in or near peak-to-peak relationship, they will damage one another. The actuators of prior clutch brake mechanisms, designed to engage the clutch elements rapidly and maintain them engaged under high force, were not well suited for cooperation with toothed clutch elements.
There are prior disclosures of clutch mechanisms having toothed clutch elements that are actuated to and from engagement by a cam actuated shifting lever. See, for example, U.S. Pat. No. 919,006 to Hancock and U.S. Pat. No. 1,476,766 to Reynolds. In each of these the cam has an impositive driving connection with the shifting lever such that the cam, by itself, cannot compel the shifting lever to move to both clutching and braking positions. In a clutch-brake mechanism, both clutch element engagement and brake element engagement must be positively controlled and precisely timed.
In theory it might appear simple and obvious to employ a cam for axially shifting an output member of a clutch-brake mechanism to defined clutching and braking positions, but with toothed clutch elements it is also necessary to provide for some yieldability in the actuating mechanism, to accommodate a possible meeting of the clutch elements with their teeth in peak-to-peak relationship, while nevertheless establishing the axially shifted clutch element accurately in its engaged position at precisely the right time in the cycle. A further complication is the need for preventing the interengaged teeth on the clutch elements from camming each other apart under normal torque forces. While a cam actuator could, in theory, rigidly confine an axially shiftable clutch element in a precise clutch-engaged position, so that no axial bias upon it would be needed, such confinement to a very precise position is out of the question for practical reasons, and the engaged clutch elements should instead be confined against separation by an axial biasing force acting on one of them to urge it towards the other.
Heretofore there has been no obvious solution to the complex problem of preventing slippage between clutch elements and achieving precise timing of their engagement. The best that could be done was to employ a powerful electromagnetic or penumatic actuator that slammed friction plate clutch elements into engagement under brute force. Frequent and costly maintenance were accepted as inevitable. Although a clutch-brake mechanism for a bag making machine or the like should move the web through an accurate distance at each advance, there should be provision for adjustingly rotating the feed rollers relative to the clutch-brake mechanism, to enable proper positioning of an imprinted web relative to the bag forming station, so that imprinted material will appear in a desired position on every bag produced. None of the above mentioned patents discloses means for accomplishing such adjustment.